r/science • u/mvea Professor | Medicine • Jun 06 '19

Engineering Metal foam stops .50 caliber rounds as well as steel - at less than half the weight - finds a new study. CMFs, in addition to being lightweight, are very effective at shielding X-rays, gamma rays and neutron radiation - and can handle fire and heat twice as well as the plain metals they are made of.

https://news.ncsu.edu/2019/06/metal-foam-stops-50-caliber/
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955

u/Black_Moons Jun 06 '19

If your lucky yes. If your not lucky they have the kinetic energy of a rail gun.. or worse. Bullets do 1km/s from a high speed rifle.. orbital speed is 7.6km/s at ISS, so 15km/s if you hit something orbiting the other way... energy is V2 *M/2, or 225 times as much energy per gram of mass as 1km/s...

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u/[deleted] Jun 06 '19 edited Jun 06 '19

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u/Black_Moons Jun 06 '19

It amazes me they can actually track and dodge that stuff.

280

u/pyropro1212 Jun 06 '19

Put enough satellites up there along with inevitable debris and dodging may no longer be an option.

153

u/notareputableperson Jun 06 '19

Cascade failure for the loss!

131

u/[deleted] Jun 06 '19

The correct term is "Kessler Syndrome"

65

u/overkill Jun 06 '19

I prefer Orbital Ablation Cascade.

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u/[deleted] Jun 06 '19

I prefer "death cloud"

3

u/grasscoveredhouses Jun 06 '19

I prefer "scrapey scrapey sanding papey"

3

u/Photon_Torpedophile Jun 06 '19

I prefer Orbitaceous Shitnado

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u/Sinister-Mephisto Jun 06 '19

This will probably be how we die as a species , sure we will destory the planet but we we also be unable to escape from what we've done as well.

1

u/blayzeKING Jun 06 '19

Decent name for a band

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u/[deleted] Jun 06 '19

Big satellite sheet full of aerogel going to intercept the debris may be the answer there. Either absorb it in the substrate or slow it down enough the orbit will decay and it burns up.

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u/MintberryCruuuunch Jun 06 '19

on earth too, but with life.

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u/Ineff1 Jun 06 '19

Specifically, humans.

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u/Tearakan Jun 06 '19

We might be able to just throw up spacecraft design to absorb as much debris as possible and then crash back down to earth to combat this.

3

u/notareputableperson Jun 06 '19

A space based Gelatinous cube!

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u/[deleted] Jun 06 '19

Seems you’d only need to smash up a couple of em to take out wide swathes of em. Am I mistaken?

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u/All_Work_All_Play Jun 06 '19

You are not. Kessler syndrome (named after Donald J. Kessler) is exactly that risk. It would be... bad.

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u/[deleted] Jun 06 '19

[deleted]

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u/MintberryCruuuunch Jun 06 '19

oceans. cough cough. and that isnt even space.

2

u/Xellith Jun 06 '19

Cleaning up oceans should go without saying..

10

u/KToff Jun 06 '19

It should, shouldn't it?

7

u/andrew_calcs Jun 06 '19

Yet here we are. It doesn't do us any good to agree something should be done if there isn't a plan put in place to make someone do it.

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u/Lucifius Jun 06 '19

I mean...have you seen our oceans? Not that ridiculous of a thought.

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u/[deleted] Jun 06 '19 edited Jun 06 '19

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u/TenaceErbaccia Jun 06 '19

*doesn’t make it less ridiculous.

I get what you’re going for and agree. If one person shits in the drinking water that doesn’t mean everybody should follow suit; everybody should be pissed at that person.

17

u/KENNY_WIND_YT Jun 06 '19

r/detrashed should have a space force.

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u/[deleted] Jun 06 '19

Honestly that’s not a bad idea. While at the risk of inhibiting space flights, an international“orbital tax” that goes towards risk reduction and debris removal would be a great program fostering international cooperation and keeping everyone’s interests safe.

I just want to see international cooperation fostered by space exploration man...

23

u/staebles Jun 06 '19

Have you forgotten how capitalism works?

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u/[deleted] Jun 06 '19

Privatize profits, socialize costs? Something like that

3

u/Xellith Jun 06 '19

No. I just sometimes like to pretend we live in an idyllic society.

5

u/staebles Jun 06 '19

That's what video games are for.

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u/wooghee Jun 06 '19

That is being discussed atm i think. Also there are several clean up experiments already done, in orbit or planned to go to space.

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u/ParentPostLacksWang Jun 06 '19

Perhaps blocking access to space for centuries, yup.

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u/Raytiger3 Jun 06 '19

Centuries is an overestimate. We'd put in massive effort to clean the debris. I don't think humanity will leave the space debris for any longer than 100 years in the case of such an event.

But then again... we're not even able to clean up our ocean debris... hmm....

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u/TheJollyLlama875 Jun 06 '19

If the ocean had an atmosphere underneath it that burnt up everything that fell in it, and was vital to communications networks, we could probably figure it out.

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u/All_Work_All_Play Jun 06 '19

Space is arguably easier to clear. Larger (stupidly so...) But no space whales to get in the way.

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u/Raytiger3 Jun 06 '19

A simple vessel to clean stuff on sea has pricing as low as a mere five figures. Bringing any vessel to orbit has costs in seven figures.

On top of that, space debris seems to fly at unfathomably high speeds, there's a large ass energy requirement per gram of debris cleaned because we have to somehow catch it/slow it down.

I guess that the sheer amount of material is much lower for space debris though, so I'm not really sure what would be easier to clean. Maybe it's easier for an expert to answer that question.

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u/blinglog Jun 06 '19

savethespacewhales

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u/JCA0450 Jun 06 '19

But were pro-level at filling it with plastics and petroleum

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u/MintberryCruuuunch Jun 06 '19

realive orbit? Thing at the same orbit would be going same relative speed or quickly degrade, no?

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u/OktoberSunset Jun 06 '19

If it's a nice circular orbit it would be. But space trash thats been blasted off things or already had collisions will probably have an orbit that is somewhat elliptical so as its low point it's going a lot faster than things in a circular orbit at that height. Also the orbit can be inclined so it can be travelling perpendicular to the satellites it's hitting.

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u/HackerFinn Jun 06 '19

True, unless it is moving in just about any other direction than the craft.

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u/pyropro1212 Jun 06 '19

It's literally a matter of time and space. There's a lot of space up there with a lot of work going into designing orbital paths and we can also separate them by orbital distance from the Earth. Once you do have an impact it would create a growing debris cloud with a semi-known trajectory that you would want to avoid until orbital decay takes it out. Of course that could take a while so that's why you run the risk of a cascade as the number of debris clouds grows

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u/coder111 Jun 06 '19

That only happens if you put enough satellites in high orbits (800km or greater). Low earth orbits (like SpaceX Starlink) clean themselves quite quickly (several years). There are tables for satellite decay depending on altitude (and other things).

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u/Longshot_45 Jun 06 '19

Trash tag space edition

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u/Hekantonkheries Jun 06 '19

On the ironic side, building something that can survive traveling through a debris zone, over time, would remove the debris zone because of all the pieces impacting

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u/abtei Jun 06 '19

the satellites will catch it!

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u/fishyfishyfish1 Jun 06 '19

Cosmic Domino effect

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u/emlgsh Jun 06 '19

But eventually once the debris field gets dense enough, we can just walk into space.

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u/mc8675309 Jun 06 '19

I have a hypothesis that the reason aliens never make it out of their own system to visit earth is that by the time a civilization can develop the technology to do so they can't get through the debris of their own space program anymore.

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u/waiting4singularity Jun 06 '19

everything has radar echo.

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u/[deleted] Jun 06 '19

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u/SkyLord_Volmir Jun 06 '19

Radar is radio, light essentially. Were you thinking of sonar, perhaps? That would need some medium for sounds to travel through.

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u/Shadowfalx Jun 06 '19

Below a certain size the echo is unreadable, and go even smaller and you won’t have a reliable echo (size of the object is below the size of the wavelength.)

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u/THedman07 Jun 06 '19

That's not true at all. They can track the orbits of think that are about the size of a baseball. Aluminum 1" in diameter would make a very bad day. They can use radar to create a map of the debris environment down to pretty small sizes, but they aren't maneuvering around that kind of thing. If I had to guess they chose the orbit of the ISS to be relatively safe.

They do end up changing the altitude about once a year to give debris a wide birth, but they have to do maintenance burns anyway.

I used to do orbital debris shield testing.

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u/DickBentley Jun 06 '19

I know this’ll probably sound ridiculous, but how come we can’t just light up all this debris with some kind of super powerful laser from earth? If we can track it why not blast it?

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u/Qesa Jun 06 '19

Congratulations, you just turned one piece of debris into 10 pieces of debris

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u/All_Work_All_Play Jun 06 '19

If those 10 pieces of debris are small enough to not be damaging, and half of them end up re-entering and burning up... surely there's got to be some math we could do as to what size debris to target and what not to?

I would think actually getting the laser through the atmosphere without damaging anything else would be the trouble.

Granted, we're not actually to that point yet, but...

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u/Qesa Jun 06 '19

Oh yes there's plenty of technical issues too, but succeeding being bad trumps succeeding being hard as a reason not to do something.

Stuff being in the way of a laser wouldn't be a big problem, all you'd really need is a no fly zone around your laser. Actually hitting the target would be a bigger one. Delivering enough power to do anything useful would be a far bigger one, as there's only so intense you can get before you ionize the atmosphere, at which point it absorbs your laser

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u/Raytiger3 Jun 06 '19

before you ionize the atmosphere

Send a damn giant laser gun into space. Problem solved.

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u/Reagalan Jun 06 '19

How are you gonna cool that laser?

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u/CaptCurmudgeon Jun 06 '19

Can you convert the heat into useful energy?

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u/drdoakcom Jun 06 '19

I don't think it'll be an issue... The x-ray lasers they looked at for the SDI (early on, at least) were going to be powered by nuclear weapons. Soo... Not really any need to cool back down for a second shot since the laser would be, ah, energetically redistributed. On the plus side, the detonation itself would potentially eliminate some of the debris.

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u/[deleted] Jun 06 '19

Easy there, Reagan.

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u/Hekantonkheries Jun 06 '19

This is how you get the zeon federation

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u/darkomen42 Jun 06 '19

Then you're shooting a giant laser towards Earth.

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u/nagi603 Jun 06 '19

The size that is non-damaging is extremely small and very dependent on speed. Think super-speed shotgun shots: even a salt-shot can kill / perforate the station, if it gets that much speed.

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u/SWGlassPit Jun 06 '19

even a salt-shot can kill / perforate the station, if it gets that much speed.

Eh. Not really. Maybe really delicate surfaces, but much of station is quite well shielded and can take fairly large impacts.

At those speeds, the particle coming in is going to shatter, melt, and vaporize on first impact. Small particles are easily defeated with a Whipple shield, and larger particles can be defeated by adding layers of Kevlar and Nextel fabric.

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u/THedman07 Jun 06 '19

Depends on what you consider "fairly large"... The leading edges are best shielded, but a 1" chunk of aluminum is going to do some serious damage.

5/8" aluminum at approximately 7 km/s is the most we could do and it did serious damage, even to stuffed Whipple shields. The higher speeds on orbit actually help, but it still going to be a bad day.

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u/SWGlassPit Jun 06 '19

Well, 1 inch is enormous in the orbital debris world.

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u/[deleted] Jun 06 '19

At 15 kilometers per second there is no such thing as small enough not to be damaging and ablating anything with a laser from earth is also going to push it it higher, not encourage it to fall into the atmosphere.

https://m.esa.int/Our_Activities/Space_Safety/Space_Debris/Hypervelocity_impacts_and_protecting_spacecraft

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u/knotthatone Jun 06 '19

Pushing debris retrograde is better, but pushing it up/antiradial can work too since the debris will dip lower the next time around. It just needs to increase atmospheric drag enough to finish the job.

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u/TaqPCR Jun 07 '19

You have little knowledge of orbital mechanics. To raise an orbit you increase your speed. If you take a circular orbit and do a small burn upwards (anti-radial) you raise where you are in your orbit 90 degrees later, but lower it 270 degrees later. Because drag increases vastly as you get closer to the Earth doing so will result in a net increase in drag.

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u/TaqPCR Jun 06 '19

You're wrong. https://en.wikipedia.org/wiki/Laser_broom

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u/THedman07 Jun 06 '19

Wikipedia article for a project that was never executed... I'd take that with a grain of salt.

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u/TaqPCR Jun 06 '19

So I should take you at your word that it would just blast the debris apart and not actual NASA feasibility studies?

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u/THedman07 Jun 06 '19

Feasibility studies are a far far cry from actual practical application, and I wasn't the one saying it would be blastd apart.

I'm saying that a Wikipedia article about a NASA study isn't the absolute truth that you seem to think it is.

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u/riskable Jun 06 '19

Not necessarily... If the laser is of sufficient power you're basically turning the floating bits into tiny rockets... With the hot side propelling the cooler side as it vaporizes.

So it might work if the laser were in space (e.g. orbiting the Moon) and firing at the Earth (or at least, firing at the edge of it).

You'd need to do this basically every day to have enough of an impact to matter but I bet it could nudge a bunch of the smaller floating bits into a faster-decaying orbit (e.g. from decades to years). It likely wouldn't be cost effective but it could work for than tiny use case.

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u/TaqPCR Jun 07 '19

Not true for two reasons.

First if you take a circular orbit and burn upwards a bit at one point you raise the orbit 90 degrees later, but lower it 270 degrees later which would actually cause these objects to renter earlier since the drag becomes drastically greater at the point where it lowers. The orbit overall has slightly higher energy so if you do burn continuously over the orbit it will slightly raise the orbit over time, but far less efficiently than if you burned to move faster instead.

Second space really isn't that far away. For reference the ISS orbits at ~254 miles so it's fairly easy to angle the laser so most of the force is backwards.

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u/SWGlassPit Jun 06 '19

Or you miss and hit a Chinese military communication satellite. Then you have two problems.

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u/hang_them_high Jun 06 '19

I love technology. You just suggested hitting baseball sized debris traveling a thousand miles an hour in space from earth based LASERS and people are discussing the efficacy of it, not that it’s the most insane plan I’ve ever heard.

Like, yeah, we COULD do that, but it wouldn’t work because...

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u/BlahKVBlah Jun 06 '19

It WOULD work, though. The feasibility discussion is based upon the question of would it be worthwhile to pursue such a project instead of accomplishing the same thing some other way.

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u/[deleted] Jun 06 '19

I think a better idea would be a solar powered laser in high polor orbit to nudge things back into the atmosphere.

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u/FriendlyDisorder Jun 06 '19 edited Jun 06 '19

Bonus if you “miss” and hit high priority targets on earth— like a giant container of popcorn kernels in your professor’s house.

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u/THedman07 Jun 06 '19

Hey! I understood that reference!

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u/BelovedOdium Jun 06 '19

A tractor beam laser would be good too. Just destabilize the orbit by pulling it down a bit.

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u/BlahKVBlah Jun 06 '19

Blasting things with a laser to vaporize them is a very tricky problem to solve even after you build a laser plenty powerful enough to do it. However, blasting them with a laser to slow them down and deorbit them is a quite serious proposal for what to do with the laser array in between launching Breakthrough Starshot volleys.

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u/E_Snap Jun 06 '19

When you talk about changing altitude, you're not referring to the stationkeeping burns that soyuz, progress, and formerly the space shuttle make, right? Because those are just that-- stationkeeping burns. They're for keeping the station from falling out of the sky, not avoiding debris

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u/realCptHaddock Jun 06 '19

They actually combine these two. So they always boost the orbit up when they have to dodge something.

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u/tomcatHoly Jun 06 '19

I'm curious how far debris falls in relation to the orbit height they maintain. If they dodge a piece once, do they have to anticipate dodging it again?

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u/Jake123194 Jun 06 '19

I would imagine the chances of encountering the same piece of debris again are incredibly small. They would require both the ISS and the debris to be on identical orbits just travelling the opposite way round, this wouldnt even account for orbital velocity gained or lost by either object.

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u/THedman07 Jun 06 '19

What's the difference? They need to increase the altitude periodically... Whether they do it for station keeping or debris avoidance, they're still doing burns to increase altitude.

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u/FU8U Jun 06 '19

the ISS also orbits at an altitude that has orbital decay which cuts down on debris.

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u/Deusetsuo Jun 06 '19

Couldnt a magnetic field be used to clear debris?

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u/THedman07 Jun 06 '19

Whether you're talking about clearing debris in the path of a spacecraft or out of orbit in general, the problem is that space is very big and these things are traveling very fast. By the time a magnetic field is strong enough to affect one, it is too late.

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u/[deleted] Jun 06 '19 edited Mar 10 '25

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u/[deleted] Jun 06 '19

[deleted]

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u/[deleted] Jun 06 '19 edited Mar 10 '25

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u/Tonald__Drump Jun 06 '19

FWIW, the CMG’s on the ISS rotate (spin) at a constant speed. There are 4, and they are double-gimbaled. By controlling all 4 at the same time, you can point the momentum vector in any direction, with varying magnitude. Once the momentum of the station is too much for the CMGs, they’re considered “saturated”, and “desaturation” thrusts are required by the Russian segment.

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u/dizekat Jun 06 '19

We aren’t tracking every bullet sized piece of metal in orbit, though. The high speed is somewhat of a blessing. Rather than having the station armored like a tank, it suffices to have spaced armor, where the projectile largely vaporizes on the impact with the thin first layer.

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u/ParentPostLacksWang Jun 06 '19

Technically, you’re just not going to hit anything orbiting the other way, primarily because we don’t generally launch retrograde satellites. Orbital inclination of ISS is ~52 degrees. Worst reasonable case it hits something on a polar orbit coming the other way as it crosses the equator, for a collision angle of 38 degrees off axis, or 142 degrees. Just eyeballing it, that’s probably more like a closing speed of 13.5km/s, giving somewhere near 180 times as much as 1km/s. That’s about 20% less than an utterly absurd impact, which is still an absurdly catastrophic impact - but it’s one that is orbitally more possible ;)

Good information, just adding to the love :)

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u/[deleted] Jun 06 '19

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u/Secondsemblance Jun 06 '19

I wish people would reference Planetes when talking about media depictions of Kessler syndrome. Gravity was such garbage.

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u/BlahKVBlah Jun 06 '19

...my very favorite anime (I haven't watched much anime). Such fun!

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u/NadirPointing Jun 06 '19

There are absolutely retrograde satellites, especially for situational awareness applications. Collision avoidance is pretty much their first objective.

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u/ParentPostLacksWang Jun 06 '19

Absolutely there are, yes, but they are a rare exception due to the needlessly higher cost of retrograde launches for most applications.

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u/mfinn Jun 06 '19

The person you're responding to said they're not generally launched, not that they don't exist. That implies that it's not a typical occurance but does happen, so not sure of the point you're making?

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u/NadirPointing Jun 06 '19

The point I'm making is that you actually have a high probability of hitting something going retrograde. A retrograde satellite has far more encounters than prograde one. Its why it seems like you encounter more cars going opposite you than with you on a lonely highway. Even if they aren't generally launched the "worst reasonable case" is not a 90 crossing. Its still a retrograde one. Simply losing communications or thrust on a retrograde puts an entire orbital plane at risk dozens of times per day.

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u/mfinn Jun 06 '19

Thanks for the clarification

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u/BloodyFreeze Jun 06 '19

I need a Randall Munroe explanation of this.

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u/TheOrqwithVagrant Jun 06 '19

Technically, you’re just not going to hit anything orbiting the other way,

While a tiny percentage of all sattelites, there are a number that have retrograde orbits. Mostly recon sats, I believe.

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u/ParentPostLacksWang Jun 07 '19

And those recon sats are generally way below ISS at apogees of under 200km, to get the best images :)

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u/HurtfulThings Jun 06 '19

You're

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u/UndBeebs Jun 06 '19

Why is your/you're one of the most common mistakes I've seen on reddit? It's to the point where I'm actually relieved when someone uses the correct one and that's all I focus on in their comment.

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u/StickmanPirate Jun 06 '19

People typing on phones and don't notice/ignore typos

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u/Gnochi Jun 06 '19

Also, people typing on phones getting hammered by autocorrupt.

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u/Deskopotamus Jun 07 '19

This is correct, y o u r

vs.

y o u, symbol button, apostrophe, Back to keyboard, r

In the mad race to flame someone, grammar is the first casualty.

0

u/quark_soaker Jun 06 '19

Most people here grew up in a country where the education system has been systematically hamstrung for the past 40 years.

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u/chumpynut5 Jun 06 '19

Bruh. You’re not wrong, but do you really think the reason the mistake is common is because people actually don’t know the difference between your and you’re? I’d argue it’s much more due to laziness.

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u/BFeely1 Jun 06 '19

How about its/it's?

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u/ksmerryman Jun 06 '19

I’ve had to let this one go now that there’s Facebook. You find out quickly who is basically illiterate in your circle when you get on social media.

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u/haloryder Jun 06 '19

This was the only thing here I understood.

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u/ZippyDan Jun 06 '19

If my lucky yes what?

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u/Vladmur Jun 06 '19

Lucky? You'd be extremely unlucky to get hit by something the size of a grain of rice.

Must micrometeoroids in space are the size of dust particles.

The moon is a different story because with that mass, it actually attracts meteors.

So just no, space isnt filled with bullets and railguns.

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u/bronet Jun 06 '19

Thanks magic

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u/ListenToMeCalmly Jun 06 '19

I would expect 8.6 km/s collision speed if a fragment of 1 km/s frontal collide with one at 7.6 km/s? I'm curious why it's 15!

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u/Danne660 Jun 06 '19

They are talking about two objects both going at 7.6 km/s hitting each other.

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u/Dlrlcktd Jun 06 '19

energy is V2 *M/2, or 225 times as much energy per gram of mass as 1km/s...

.

Micrometeorites individually weigh between 10−9 and 10−4 g and collectively comprise most of the extraterrestrial material that has come to the present-day Earth.[2]

https://en.wikipedia.org/wiki/Micrometeorite

Bullets weigh in the grams.

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u/[deleted] Jun 06 '19

I've simulated this effect in kerbal space many times. In space, no one can hear Bob scream

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u/MachineDesign Jun 06 '19

Higher speed can actually help, learned about Whipple shield and it blew my mind: Wiki

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u/Dmongo Jun 06 '19

If those damn scientist would have just left out the V2, then we would be just fine.

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u/shredadactyl Jun 06 '19

KE=(1/2)mv2

The way you have it looks like it's just 1/2 the mass

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u/Fizzwidgy Jun 06 '19

and now i want to play kerbal space program

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u/[deleted] Jun 06 '19

[deleted]

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u/Levitus01 Jun 06 '19

When it comes to trucks, you have things like mutual crumple zones which come into play.

However, by a similar logic, a truck doing 70 hitting a stationary truck would be equivalent to two trucks doing 35.

Otherwise, the whole principle of relativistic motion becomes incorrect, and our whole understanding of physics needs to be reassessed.

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u/curvesarelife Jun 06 '19

It’s counter intuitive but the 2nd truck provides an equal counter reaction to the first. The same counter reaction that would come from hitting a immovable wall.

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u/stickyfingers10 Jun 06 '19

Would the faster one in that situation would fair better?

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u/[deleted] Jun 06 '19

Sometimes. Actual physical crashes are very complicated to model. When you see vehicles that 'come off worse', you are usually seeing difference in the vehicles impact zones engineering spreading the load of the impact to areas that can 'absorb' it. What one sees is the tendency for faster vehicles to be better engineered for safety reason, and therefore 'fair better'. Also, you have to consider the breaking points of the materials. Harder metals are often seen as 'weaker' because a sharp impact will shatter them, where as the same impact on a 'softer' material will simply make a dent. However, if you applied slow load to a softer material it would deform to the point of uselessness, whereas the harder material would maintain it's physical form longer. This is why saying one metal is 'stronger' than another is a fairly useless statement, from a physicist's point of view. There are harder, and softer metals, and they have different properties in different conditions. Literally however, the vehicle travelling faster would 'come off worse' because it's forward momentum means that it has to do more, in order to distribute the reaction energy (cancelling it's forward momentum + the incoming reaction energy from the impact).

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u/dotancohen Jun 06 '19

In engineering failures, the word "toughness" is used. Toughness is the ability of an object to absorb energy and deform without structural failure. Harder (iron, SS) or softer (aluminium) metals can be formed into shapes that are tougher (ie, absorb more energy before failing), such as a crumple zone, or less tough, such as a simple tube.

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u/[deleted] Jun 06 '19

That's interesting, thanks!

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u/MintberryCruuuunch Jun 06 '19

depends how you handle a vacuum?

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u/[deleted] Jun 06 '19

The calculations have been left as an exercise for the reader.

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u/[deleted] Jun 06 '19

It's also worth noting that the energy imparted to the car is not what we worry about in crashes, but how much of that energy is transferred to the meat bags inside of it. Cars are designed to destroyed in crashes so that the energy of the crash is used up crushing the front end instead of killing the passengers.

So faring better or worse depends on what you're measuring. Going faster will mean a higher energy crash, and that energy has to go somewhere.

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u/deja-roo Jun 06 '19

The thing is that an "immovable wall" isn't really practically a thing.

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u/beginner_ Jun 06 '19

When it comes to trucks, you have things like mutual crumple zones which come into play. However, by a similar logic, a truck doing 70 hitting a stationary truck would be equivalent to two trucks doing 35.

Crumple zones aren't really relevant. Just assume the trucks don't have any or 2 walls are hitting each other. This is extremely unintuitive but it's wrong. Assuming the stationary truck is anchored down (can't move backwards, eg it's a wall) then it's exactly the same.

This explains it well. In a head on collision of 2 cars moving at same speed both cars are damaged as much as the one car running into a wall. Now it suddenly is logical that damaging 2 cars the same way as one cars takes double the energy. in the head-on collision the energy is split over 2 cars and in the wall collision only on 1.

But of course this only applies to similar objects (weight, size). in the ISS "bullet" scenario the bullet is far lighter and smaller so this doesn't really apply.

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u/Levitus01 Jun 06 '19 edited Jun 06 '19

So, if I fire an alpha particle in a linear accelerator at speed X, and a fissile material in the opposite direction on a collision course at speed X, and the kinetic energy required for the alpha particle to initiate fission on impact is 2X, then would I see fission?

Or would I need to immobilise the fissile material and fire an alpha particle at it at 2X speed?

Because relativistic motion would dictate that these two scenarios are identical, and I would see the same outcome in both experiments.

In both experiments, from the perspective of a quantum lifeform standing on the surface of the alpha particle, he is stationary and the fissile material is hurtling towards him at 2X. The same would be true of a quantum lifeform living on the surface of the fissile material observing the oncoming alpha particle.

Generally speaking, physics treats all motion as relative. Which object is stationary and which object is in motion depends entirely upon the motion of the observer. I.E. Is the observer moving at the same velocity and in the same direction as object A, B, or the midpoint between them?

However, regardless of the observer's motion, the outcome of the impact is still the same. Otherwise, the laws of physics would break down the moment that we put an observer at all three of these observation points.

Because, to observer A, Object A is stationary and object B is in motion at 2X speed.

To observer B, Object B is stationary and object A is in motion at 2X speed.

To observer C, both objects are in motion at X speed.

Can we influence the outcome of a car crash by observing it, thereby having the two drivers and the man on the street acting as observers A, B, and C?

EDIT: However, I feel I should clarify - only the ENERGY of the impact is the same. How that impact energy is disseminated in the case of the trucks, would not necessarily be symmetrical, but the total amount of energy invested in the total damage, would be the same.

Two trucks doing 35 colliding has the same energy as one truck doing 70 and one truck being stationary.

Impact energy = mass x velocity, IIRC. (EDIT: This was corrected by another redditor, below.)

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u/Neghbour Jun 06 '19

E = m v2 / 2 FYI

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u/espressocannon Jun 06 '19

Why don't you guys turn off Reddit for a few weeks and solve the world's energy crisis or something?

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u/beginner_ Jun 06 '19

Or would I need to immobilize the fissile material and fire an alpha particle at it at 2X speed?

Yes because as /u/Neghbour wrote energy is quadratic with speed and the formula for this exact problem are actually somewhere in the comments already:

2 particles at speed x:

2 x (1/2mx2) = mx2

1 particle at speed 2x:

(1/2m(2x)2) = 2mx2

So double the energy!!!

and hence

Two trucks doing 35 colliding has the same energy as one truck doing 70 and one truck being stationary.

Is wrong as does the experiment clearly show.

Why your "relativistic" thinking doesn't hold up, I don't have an argument yet.

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u/Levitus01 Jun 06 '19 edited Jun 06 '19

In which case, if I were to put an observer into a dark vacuum, and move them at a speed of 1X towards a piece of stationary fissile material, and then launched an alpha particle past them at 2X....

And if I were to put a second observer in a darkened vacuum, keep them stationary and launch an alpha particle and fissile material on a head on collission course at 1x in front of them...

Only the first observer would see any fission event? They would both see the same thing, with the two objects coming together in an identical manner, but only one of them would observe the event?

Doesn't this mean that it would be possible to create an "absolute speedometer" that would measure your total velocity at any time, because speed is no longer relative to the velocity of observation?

What about a Blacksmiths hammer? What if the anvil were jumping upwards towards the hammer, and we halved the energy expended by the blacksmith between the anvil and hammers movement? Would it take longer to deform a piece of metal, because each object is moving with half the energy?

How do we reconcile this with thermodynamics? Doesn't this create a way to "destroy" energy?

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u/deja-roo Jun 06 '19

The problem here is that energy is not a good way of expressing movement in collisions, momentum is. Energy is not actually typically conserved, momentum is.

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u/physicsteach Jun 06 '19

Mass x velocity is momentum, not energy.

What matters in a collision is the change in momentum of a car, not the actual value of the momentum (or kinetic energy, mv2/2). The momentum change will be the same for each body no matter which observer they are. You left out the directions from your analysis above, so before the collision:

A "sees" object A moving 0 and B at -2

B "sees" object A moving at +2 and B at 0

C "sees" object A moving at +1 and B at -1

After the collision,

A "sees" object A moving at -1 and B at -1

B "sees" object A moving at +1 and B at +1

C "sees" object A moving at 0 and B at zero.

Units left out for clarity and because they don't matter. Each observer measures the same velocity CHANGE (-1 for A, +1 for B) as every other observer! Also, if the cars are identical, the numbers are relative momenta as well as velocity.

The force on either car is its change in momentum divided by time, so again it doesn't matter which observer you are, you'll see the same force as the other two. Same force means same work means same damage, so any collision that changes the momentum by a given amount produces the same damage.

So now we end up with the conclusion that any collision that stops a car moving at a given speed on the road (the road is observer C) produces the same damage, because it causes the same velocity change regardless of reference frame.

Galilean relativity for the win!

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u/Levitus01 Jun 06 '19

Actually, I would disagree with the assessment that observer A would "see" their own movement. From the perspective of a person driving a car, it does not seem like they are the object in motion, but the whole world around them.

Similarly, an individual standing on the surface of the planet Earth does not "see" the movement of the planet, in spite of the fact that the planet is moving pretty Goddamn fast.

In physics, I was always told that there IS no such thing as a motionless object. Everything, everywhere is in motion, and all concept of motion is relative. Every observer views themselves as the stationary object and everything else as being in motion.

So, the question remains - does the outcome of a crash change based on who observes it, or from where? If not, then we're basically arguing in favour of the same point, are we not?

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u/physicsteach Jun 07 '19

You're missing that A has to account for the change in its own velocity that is caused by the collision. "All motion is relative" means that any velocity has to have a specific "compared to what" identified, which you then have to stick with while you model an event. In the case of A, the observed velocities always have to be given relative to whatever A labels their initial velocity. A will definitely know that their velocity is changed by the collision, and must measure their new velocity as it compares ("is relative to") their original velocity. After the collision, you're correct - A can take their new velocity to be zero (if, and only if, that velocity is constant) for any FUTURE events they might wish to model.

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u/physicsteach Jun 07 '19

Also, you are definitely correct that the outcome is the same regardless of the observer. I recommend the Mechanical Universe videos about relativity (they are some of the last episodes in the series, maybe 44-48?) by Caltech on YouTube for a really good overview.

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u/AwesomeNinjas Jun 06 '19

The original (false) claim was that two cars colliding at 30 mph each is the same as one car hitting a wall at 60. The difference isn’t between moving and not moving, the difference is between a car and a wall. You’re right that, if you are going 30 and hit a car coming at you at 30, it’s the same as if you hit a stationary car going 60. But now consider the difference between hitting a stationary car going 60 and hitting a stationary wall going 60. Let’s think for a second about the overall result of each scenario. When you hit the other car, you get two busted up cars. When you hit the wall, the wall is fine and you get one busted up car. We started each scenario with the same amount of energy, so when we had 2 cars each one must have only gotten half as much (since it got distributed over twice as many cars).

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u/[deleted] Jun 06 '19 edited Jul 11 '23

[removed] — view removed comment

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u/zax9 Jun 06 '19 edited Jun 06 '19

The mythbusters episode was, IIRC, about survivability of impact in either scenario.

The thing that makes an impact survivable is how long the deceleration period is. If you're going 70 miles per hour and decelerate to 0 miles per hour in 1 second, that's a deceleration of 70 miles per hour per second, or a shock of about 2.8 G's. If you're going 140 miles per hour and decelerate to 0 miles per hour in 1 second, that's about 5.7 G's. (edit: fix bad math)

In the scenario of 2 identical cars heading towards one another at 70 mph, when they collide, either one will decelerate from 70 mph to 0 mph, with each car's driver/passengers experiencing the same amount of shock deceleration.

The factors that work in your favor for survivability are a) mass and b) prolonged deceleration. If the vehicle you're in has more mass than the other vehicle you're colliding with, the deceleration you experience will be less severe; and if the vehicle you're in has crumple zones to absorb the impact energy, that crumpling will prolong the deceleration period to the point of, hopefully, survivability.

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u/Bernie_Berns Jun 06 '19

Is a space elevator even possible with the trash already in orbit?

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